a. Field of the Invention
The invention relates to an economizer for preheating feed or return water in a high pressure steam or hot water fire tube boiler having a smoke tube connection-effecting portion.
b. Prior Art
In known boilers comprising large water chambers and a smoke tube connection-effecting portion, e.g. flue/smoke tube boilers for high pressure steam and hot water, the boiler feed water and return water respectively are supplied to the water chamber of the boiler, wherein they mix with the hot boiler water, the natural or connective circulation of which within the boiler is determined by the heat transfer on the various parts of the heating surface.
Since the cooling or the smoke passing through a smoke tube, or in other words the heat transfer from the tube, is essentially determined by the temperature of the surrounding water, the outlet temperature of the smoke of such known boilers varies according to the temperature of the boiler water. The boiler is often constructed in such manner that the smoke, at maximum load, is cooled to a temperature of about 100.degree. C. above the temperature of the boiler water, i.e. to the water temperature in the steam boilers corresponding to the saturation temperature at the boiler pressure, or to the water flow temperature in the hot water boilers. This difference of about 100.degree. C. between the temperature of the smoke and the boiler water ensures an appropriately high specific heat transfer across the last section of the heating surface which effects natural circulation or convection of the boiler water. However, the heat content in the flue gases is normally not utilized sufficiently, i.e. the smoke heat loss (chimney loss) is too high and consequently the boiler efficiency too low.
Known boilers have to some extent solved this matter by various means.
In some of said boiler designs the smoke cooling is improved by increasing both the proportion between the length and bore of the smoke tubes, and the total heating surface of the smoke tubes. The heating surface is for instance increased by 25% so as to obtain cooling of the smoke to the temperature of the boiler water +50.degree. C. instead of +100.degree. C.
In other boilers, the smoke tubes are provided with turbulators of various designs. In this manner an improved cooling of the flue gas is obtained as a consequence of the higher smoke velocities etc., compared to corresponding tubes without turbulators. However, a significant smoke tube advantage is simultaneously lost, i.e. the convenience of complete inspection and cleaning of the smoke side of the boilers.
In both these known boiler types, the outlet temperature of the smoke varies with the temperature of the boiler water.
Other known boilers are provided with an economizer comprising plain or ribbed tubes so as to obtain an improved cooling of the flue gases. Such an economizer implies that the outlet temperature of the smoke varies according to the temperature of the supplied feed or return water respectively. However, several significant smoke tube advantages are simultaneously lost, i.e. the possibility of complete inspection of both the smoke and water side, and in particular the insensitivity towards moderate deposits on the water side, which is typically in smoke tube boilers.
In some known older flue/smoke tube boilers "the locomotive type boilers" the water chamber, cf. for instance Norwegian patent specification No. 90290, is provided with a screen plate, which together with the boiler shell and the rear boiler plate form a preheater space for the feed and return water respectively for steam and hot water boilers (and "water heaters").
The movement of the water in such preheater spaces of a large volume is determined by the natural circulation of "cold, downward flows" passing around the smoke tube section and "hot, upward flows" passing through said section. The average water temperature at the smoke tube heating surface is consequently relatively high, and the economizing effect of the preheater space, i.e. the cooling of the flue gas, is correspondingly low. For typically permissible feed and return water temperatures and typical steam pressure and hot water temperatures respectively, the efficiency has thus been improved by up to about 1% for steam boilers and up to about 1.5% for hot water boilers without condensing steam in the preheater space.